Various different types of sensors are known for use in determining the yield in harvesters. Examples are baffles or rotating star feeders at the outlet of the grain elevator of a combine as disclosed in EP 0 208 025 A and U.S. Pat. No. 6,584,424B or baffles inside the delivery channel of a field chopper as disclosed in EP 0 887 008 A, weight sensors for capturing the weight acting on crop material carrying surfaces as disclosed in U.S. Pat. Nos. 4,407,380A, No. 5,318,475A, No. 5,959,257A, No. 6,066,809, and No. 6,313,414B, sensors for capturing the distance of prepress rollers as disclosed in DE 195 24 752 A, light barriers inside the combine elevator as disclosed in EP 0856 723 A, or radiometric absorption measurements at the elevator outlet as disclosed in WO 85/00087 A. In the case of mechanical sensors the crop material moves along a surface or strikes it, which generates forces, pulses or torque which can be transduced into electrical signals by potentiometers or according to EP 0 856 723 A by extensometers or optical displacement sensors. In DD 267 650 A a piezoelectric crystal is used for transforming the weight of the crop material acting upon it into an electrical signal.
By means of a speed measurement of the material the material throughput per time unit can then be determined. If the density of the crop material is known, the volumetric throughput can then be deducted.
The sensors mentioned have the disadvantage that additional elements need to be integrated into the harvester. Also, their accuracy and reliability is not always adequate in the highly dust and vibration impacted environment of a harvester.